Apparatus and process

ABSTRACT

A process for preparing fibrous sheets of organic synthetic polymers in which a filamentary web is entrained in a gaseous stream flowing in a path toward a receiving surface and the web is electrostatically charged before being collected, the improvement being confining the gaseous flow before charging by directing it through a critically dimensioned passage which converges in the direction of flow. The apparatus includes an element which together with the target plate of the charging device for electrostatically charging the fibers structurally defines the passage.

United States Patent {72; Inventor Lawrence lsakoff Wilmington, Del. [21] Appl No. 886,967 I32] Filed Dec. 22, 1969 [45] Patented July 13, 1971 l 7.1] Assignee E. l. du Pont de Nemours and Company Wilmington. Del.

[54] APPARATUS AND PROCESS 6 Claims, 13 Drawing Figs. [52] U.S.C1 317/262 A, 156/167, 156/272, 156/380, 264/24, 18/8, 19/155 [51] lnt.Cl D04h 5/00 [50] Field of Search 317/3, 4, 262 R, 262 A; 264/24; 156/167 [56] References Cited UNITED STATES PATENTS 3,387,326 6/1968 Hollberg et a1. 18/8 FOREIGN PATENTS 1,134,685 11/1968 GreatBritain Primary Examiner-Lee T. Hix Attorney-Howard P. West, Jr.

ABSTRACT: A process for preparing fibrous sheets of organic synthetic polymers in which a filamentary web is entrained in a gaseous stream flowing in a path toward a receiving surface T0 SOLVENT RECOVERY i 1 GROUNDED 0R [)0 E OF OPPOSITE R0 POLARITY T0 35 M PATENTEUJUL 1 3I97l 3, 593; 074

T0 van g" RE ERY POLYMER SOLUTION f/l/A/l DC SOURCE OF POSITE POLARITY T0 35 A 7' Q INVENTOR LAWR E NC E ISAKOFF ATTORNEY PATENTEUJULBISYI 4 3- 4 sum 2 or 3 41; FIG. 7 m 4 6" 1g" 7 FIG.6

I I x n 'NVENTOR 45" LAWRENQE'ISAKOFF BY W ATTORNEY FIG. 8

PATENTED JUL 1 319?:

SHEET 3 OF 3 INVENTOR LAWRENCE ISAKOFF APPARATUS AND PROCESS BACKGROUND OF THE INVENTION This invention relates to a process and apparatus used in the preparation of nonwoven fibrous sheets of synthetic organic polymers. More particularly, it is directed to a process and apparatus for spreading a plexifilamentary strand into a planar web, directing the web towards a surface, charging the web and collecting the web in the form of a nonwoven fibrous sheet having improved uniformity over that obtained by prior art methods.

In the preparation of fibrous nonwoven sheets, various methods and apparatus have been developed for dispersing the filaments from a bundle into a wide band and for directing the web by oscillating means in a programmed manner to various locations across the width of a moving collecting surface. For example, the Steuber patent U.S. Pat. No. 3,169,899 describes a process for making a nonwoven sheet from flashspun fibrous materials. In the flash-spinning technique a solution of an organic polymer which is under pressure and at a temperature far above the boiling point of the solvent is extruded into an area of substantially atmospheric pressure. As the material issues from the orifice, the solvent expands rapidly and a plexifilamentary strand is formed. The plexifilamentary strand is composed of very thin film-fibril elements which are interconnected in a three-dimensional network as described in detail in Blades & White U.S. Pat. No. 3,081,519. The three-dimensional network is spread into a wide web by causing it to be swept along a smooth path past a curved surface whereupon the expanding solvent gas spreads the material. The oscillating motion serves to direct the web to various areas across the width of a moving collecting belt where it is deposited in the form of swaths. The web can be electrostatically charged to increase its width and increase the separation between the fibrils. A fibrous nonwoven sheet is thereby obtained.

In an alternate process described in copending U.S. Pat. application Ser. No. 628,871 filed Apr. 6, 67, now U.S. Pat. No. 3,497,918 the oscillating baffle can be replaced by a rotating bafile, having specially contoured surfaces, which simultaneously spreads and oscillates the web as it is directed through an electrostatic device to apply uniform electrostatic charge on the web and promote unifonn deposition of the web on a moving collecting surface. A particularly advantageous charging apparatus is described in Kilby and Smith, U.S. Pat. No. 3,456,156. The apparatus consists of an annular disc target electrode which is concentric with the rotating baffle and rotates independently of said baffle. A multinecdle ion gun is positioned opposite the target plate, the needles being aimed at a portion of the target electrode to provide a corona discharge zone. The fibrous material moving in a planar path between the target electrode and the ion gun needles is electrostatically charged before being deposited on the moving collecting surface.

A number of requirements must be satisfied in order to obtain wide, fibrous, nonwoven sheets having a uniform appearance and a uniform fabric weight. In general, wide nonwovens are obtained by blending and overlapping the output from several spinning positions. Copending U.S. Pat. application Ser. No. 628,872 filed Apr. 6, 67, describes a mechanism for making fine adjustments and varying the weight distribution of the webs deposited on the collection surface. Tests have shown that optimum fabric weight uniformity in the cross-machine direction (i.e., the direction at right angles to the direction of movement of the receiving surface) is obtained when the width of the swath at this surface is within certain limits which depend on the shape of the cross-machine direction fabric weight profile produced by each spinning position. This width is a function of the amplitude of the oscillation imparted to the web by the baffle, the amount of electrostatic charge on the web and the distance between the baffie and the receiving surface. Within certain limits, the wider the swath at the receiving surface the easier it is to obtain a sheet having uniform fabric weight in the cross-machine direction.

Uniformity of fabric weight in the machine direction is also important and it has been found to be deleteriously affected by an uncontrolled random oscillation of the web in a direction at right angles to that imposed by the baffle, i.e., in the machine direction. This uncontrolled oscillation, presumably due to gaseous turbulence around the web, also contributes to a surface defect on the sheet termed fiber swirl which detracts from its visual appearance. This random oscillation also increases the number of other defects such as folds, twists, or pleats in the sheet which produces localized areas of high fabric weight. This reduction in sheet uniformity becomes more severe as the amplitude of the uncontrolled oscillation increases and hence as the distance from the baffle to the receiving surface increases. Decreasing the distance between the baffle and the receiving surface improves the machine direction uniformity, however, this simultaneously decreases the width of the swath (in the cross-machine direction) at the receiving surface which reduces the blendability of the individual swaths and worsens the fabric weight uniformity of the sheet in the cross-machine direction.

It has been found that we spinning throughputs are increased the larger volumes and velocities of gas produced in the flashspinning operation create an undesirable increase in turbulence. This increases the random oscillation of the web producing a nonwoven sheet having less than the desired uniformity.

The object of the present invention is therefore to provide a process and apparatus for the production of nonwoven sheets of plexifilamentary materials having improved uniformity. In particular, the process and apparatus provides such sheets having heretofore unattainable uniformity at high spinning throughputs.

SUMMARY OF THE INVENTION The apparatus of the invention comprises means for forming a filamentary strand entrained in a gaseous stream, means for spreading the strand into a web and directing it in a plane towards a receiving surface, a short critically dimensioned confining passage which converges in the direction of movement of the web and which is formed between two surfaces ha ng no direct contact with one another, means for applying an electrostatic charge to the web and a receiving surface for collecting the web in the form of a nonwoven sheet. The confining passage provides a spreading action on the gaseous flow by decreasing the velocity component in the direction of web movement and increasing the velocity component in the directions perpendicular to the web and parallel to the width direction of the passage. This spreads the web further and decreases gas turbulence in the direction of web movement to produce a nonwoven sheet having improved uniformity.

The apparatus is used in conjunction with a process for the preparation of nonwoven fibrous sheets which comprises flash spinning a plexifilamentary strand in a generally horizontal direction, deflecting the strand into a generally vertical plane downwards towards a receiving surface by passage along the curved surface of a baffle member, spreading the strand into a web and oscillating said web in the generally vertical plane, passing the web through the converging passage and then between an ion gun and a grounded target plate to impart an electrostatic charge on the web and collecting the web on the receiving surface in the form of a nonwoven sheet.

The converging passage is formed by positioning a scoop element opposite the target plate above the point of application of the charge. The scoop element comprises a surface having an upstream edge and a downstream edge which, together with the target plate, define the inlet and outlet to the passage. Looking downwards into the passage, the shape of the outlet is defined by the distribution of distances between the downstream edge of the scoop and the target plate. The

passage has a length dimension measured in the direction of movement of the web and a width measured in the plane of the target plate in a direction at 90 to the direction of movement. This width should be greater than that covered by the oscillation of the web at the exit of the passage.

The dimensions of the outlet to the passage are critical. When the distance between the downstream edge of the scoop and the target plate is too small, the web will plug the passage. If the distance is too large, there is no confining effect and no accompanying improvement in the uniformity of the product. Considering the projection of the outlet to the passage onto a horizontal plane, the minimum distance D between the target plate and the downstream edge of the scoop should be at about the midpoint of said edge and should be equal to or greater than 0.03 inch and in the range 0.030.25 inch. When this distance exceeds about 0.5 inch, the sheet uniformity is comparable to or may be worse than with no scoop present.

The other critical dimension is D,, measured between the downstream edge of the scoop and the target plate at points symmetrically disposed about the midpoint of said edge. The points on the edge at which Dis measured are located, with reference to the projection of the outlet on the horizontal plane, by drawing a circle having a radius of 1 inch and its center on the midpoint of the projection of the downstream edge on the plane and determining the points at which this circle intersects the projection of the downstream edge. D is chosen such that DJD is in the range I3.2S.

The angle at which the passage converges in the direction of movement of the web is preferably between about 10 and 60 and the length of the passage in this direction is preferably between about 0.5 and 3 inches. The scoop element may have various shapes which depend on the shape of the target plate. Preferably, the scoop has a wedge-shaped cross section with the downstream edge being thinner than the upstream edge to provide uniform and minimum aerodynamic turbulence during operation. In a more preferred embodiment, the scoop element is pivotably mounted and spring-loaded, as hereinafter described, to provide automatic relief of partial plugs.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a cross sectional elevation indicating schematically the arrangement of various elements of an apparatus which can be used in the practice of the invention.

FIG. 2 is a view seen from the top, of a confining, converging passage used in the process of the present invention.

FIG. 3 is a cross section taken along the line 3-3 ofFIG. 2.

FIG. 4 is a view of another converging passage used in the process of the present invention.

FIG. 5 is a cross section along line 5-5 of FIG. 4.

FIG. 6 is a front elevation of a semicircular scoop which can be used with a circular target plate.

FIG. 7 is a plan view of the scoop in FIG. 6.

FIG. 8 is a cross section along the line 8-8 of FIG. 6.

FIG. 9 is a front elevation showing the scoop of fig. 6 positioned opposite a circular target plate and a rotary baffle.

FIG. 10 is a cross section along the line 10-10 of FIG. 9 with some of the elements removed for clarity.

FIG. 11 shows the operating principle of a scoop designed to open quickly in the event ofa plug.

FIGS. 12 and 13 are a front and side elevation, respectively, of an assembly drawing of a scoop incorporating the quickrelease principle.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. I, a spinneret device 1, is shown connected to a polymer solution supply source. Polymer solution 2 under pressure is fed through an orifice 3 into intermediate pressure or letdown pressure zone 4 and then through spinning orifice 5 into web forming chamber 6. The extrudate from spinning orifice 5 is a plexifilament 7. Due to the pressure drop st spinning orifice 5 and the high temperature of the spinning solution, vaporization of solvent creates a vapor blast which, by passage along the surface of baffle 8 concomitantly with plexifilament 7, generally follows the path of advance of the plexifilament 7 from spinning orifice 5 to collecting surface 9, thereby creating a flow pattern within chamber 6. Baf fle 8 is mounted on shaft 10 which in turn is oscillatably mounted in bearing 11 and is powered to oscillate by means not shown. While oscillating of the baffle is not essential, it is preferred for the preparation of wide sheets.

Target plate I3 and an ion gun M are disposed on opposite sides of the path of advance of the plexifilament web downstream from the baffle 8 and scoop element 45 is positioned above the charging zone on the same side of the path of advance as gun I4. Surface 48 of scoop element 45 in cooperation with plate I3 forms a confining passage which will be described in more detail hereinafter.

Target plate I3 is so disposed that the vapor blast originat ing at 5 and the airflow pattern in chamber 6 carry plexifilament 7 along its charging surface. Target plate 13 is connected to ground by wire 15 and microameter I6 which indicates target plate current. The location of ion gun I4 is schematically illustrated along the path of advance of plexifilament 7 during operation. The ion gun may be suspended from the ceiling of chamber 6 or from spinneret device II or mounted on brackets to the wall of chamber 6.

After passing through the charging zone, plexifilament 7 is deposited upon a collecting surface 9. The surface illustrated is a continuous belt forwarded by drive roll 36. The belt is grounded, or power source 37 is used to give it an opposite electric charge to that imposed on plexifilament 7 in the charging zone. Due to differences in their electrostatic charge, the plexifilament 7 is attracted to surface 9 and clings to it in its arranged condition as a sheet 38 with sufficient force to overcome the disruptive influences of whatever vapor blast may reach this area. Surface 9 carries sheet 38 out of chamber 6 through port 39. Flexible elements 40 across port 39 and also across port 41 which permit re-entry of the unloaded continuous belt, assist in retention of vapor within chamber 6. The sheet is then lightly compacted by compacting roll 44 and is collected on windup roll 42. A conventional solvent recovery unit 43 may be beneficially employed to improve economic operation. Wide sheets are produced by blending and overlapping the output from several spinning positions placed in an appropriate manner across the width of a receiving surface such as the belt 9.

FIGS. 2 and 3 represent one embodiment of the confining passage of this invention. FIG. 2 is a view from above showing the target plate I3 and the scoop element 45, having an upstream edge 46 and a downstream edge 47 as boundaries for surface 48, in facc-to-face relationship. FIG. 3 is a cross section taken along the line 3-3 of FIG. 2 showing the scoop element positioned at an angle relative to the target plate I3 thereby forming the converging and confining passage between plate 13 and surface 48 of element 45. In this particular embodiment the scoop is a flat plate having a wedgeshaped cross section and the minimum distance D is measured at the midpoint of the downstream edge 47 while the distance D is measured along this edge at points 1 inch on either side of the midpoint. With this arrangement, D must equal D Forcing the web and its enveloping gaseous stream to pass through this confining passage diverts a portion of the gas from its mainly downward direction to the horizontal direction as shown by arrows in FIG. 2. This slows down the velocity of the web in the direction of the receiving surface and dampens down vibrations while simultaneously increasing the width of the web as it emerges from the passage.

FIGS. 4 and 5 represent a different embodiment of the confining passage of this invention. In this embodiment, the flat plate 45 is replaced by a curved plate 45', thus forming a passage having its narrowest point at the center and widest point at the edges of the plate 45. Again D is measured at the midpoint of edge 47 and D (greater than D in this case) is measured at points on edge 47' symmetrically disposed about the midpoint and having a straight line distance of 1 inch from the midpoint.

FIGS. 6, 7 and 8 are different views ofa scoop which can be used in a more preferred embodiment of the apparatus and process of this invention, the assembly being shown in FIGS. 9 and 10. In this embodiment, the oscillating baffle 8 in FIG. 1 is replaced by a multilobed, rotating baffie 8' of the type described in copending U.S. Pat. application Ser. No. 628,871 filed Apr. 6, 67. Associated with the rotating bafile is a rotating annular target plate 13'. An ion gun (not shown) is located in proximity to the path of the fibrous material and has a multiplicity of needles disposed across the width of the path and pointing towards the path, the needles being located in a plane generally parallel to the path to form a charging zone 49. Details of this are given in copending U.S. Pat. application Ser. No. 628,868 filed Apr. 6, 67. The outlet to the confining passage has its narrowest point (D at the midpoint of the downstream edge 47" of the scoop element 45". D is again measured at a straight line distance of 1 inch from this midpoint.

A quick release scoop is one which automatically moves away from the target plate to allow a mass of material which is beginning to plug the passage to pass through the scoop gap and then returns to its original position. One method of accomplishing this is illustrated in FIG. 11 which shows a springloaded, pivoted scoop. In operation, the main flow of gas downward 50 provides a component of force 51 which tends to move the scoop element 45 away from the target plate 13. The counterbalancing force 52 provided by spring 53 keeps the scoop element in place against the stop 54. If plugging begins, the gas force increases and overcomes the counterbalancing force to move the scoop. The mass of plugging material passes through the increased gap, the gas force decreases and the counterbalance force moves the scoop back to its normal position. To insure stable scoop clearances throughout minor gas force variations, the scoop is slightly overloaded against the fixed stop 54.

The counterbalance force to be provided by the spring is a function of the scoop gap and the amount of gas generated in the spinning operation and can be determined by experimentation. Negator constant tension springs (e.g., of the type manufactured by Ametek Corp. of Hatfield, Pa. have been found particularly suitable for applying this force. Under the conditions of Example 4, the use of springs which provide a counterbalancing force of about 1.3 lbs. at the midpoint of the scoop has been found satisfactory.

FIGS. 12 and 13 are a front and side elevation ofa scoop assembly incorporating the quick-release features described above and which is conveniently attached to the bracket or other device holding ion gun 14 (FIG. 1). Two springs 53 are used to provide the counterbalance force one on either side of the scoop. As shown in FIG. 13, one end of the spring is attached to the bracket at 56 and the other end is attached to the scoop at 57. Screws are used as the stops 54 to permit adjustment of the scoop clearances. The pivot is provided by a cylindrical pin 55, one end of which is cemented to the vertical extension 58 of the scoop element and the other end being carried in a bearing 59 fixed in support 60.

In the following examples which illustrate the invention, the improvements in fabric weight uniformity and reduction of sheet defects are measured in the following manner:

PERCENT CV OF FABRIC WEIGHT UNIFORMITY A sheet of material about 500 inches long and at least 8 inches wide is used. Eighty l-inch diameter circles are cut from the sheet along three rows, the center-center distances of these circles being about 3 inches in the width direction of the sheet and 6 inches in the length direction. The coefficient of variation (percent CV) ofthe weights of these 1 inch circles in each row is calculated and the average percent CV for the three rows is used as-a measure of the sheet uniformity.

In order to make comparisons between materials having different fabric weights, a corrected percent CV is calculated which expresses the results in terms of a 2 oz./yd. sheet. This is calculated from the measured percent CV in the following manner: percent CV (correeted)=pervent CV measured ygefai'i Low values of the percent CV (corrected) indicate better sheet uniformity.

WEB DEFECTS PER CYCLE MACHINE. DIRECTION (MD) MOTION This is the maximum distance covered by the web in the machine direction during a web cycle. The average value is obtained by making this measurement over 12 web cycles and dividing the sum by 12. The lfll aesthetics of the sheet are improved the lower the MD motion.

EXAMPLE I A series of sheets from a plexifilamentary material were prepared using an apparatus similar to that in FIG. 1, having a single flash spinning position and modified as in FIG. 9 to use a circular target plate and a trilobal rotating baffle. In some of these experiments, a scoop of the type shown in FIGS. 6-8 was used at various settings of the distances D, and D /D Linear polyethylene having a density of 0.95 g./cc. and a melt flow rate of 0.9 gram/l0 minutes as determined by ASTM method D123857'I, condition E, is flash spun from a 12.6 percent solution in trichlorofluoromethane. The solution is continuously pumped to the spinneret assembly at a temperature of 186 C. and a pressure of about 1600 p.s.i., at a pUlyIflCl' throughput of 35 lbs./hr. The solution is passed through a first orifice (L/D 0.025/0.035 inches) to a pressure letdown zone where the pressure is reduced to 1050 p.s.i. and finally into the surrounding atmosphere through a second orifice (L/D: 0.025/0.03O inches). The resulting plexifilamentary strand passes along the surface of a rotating baffle which simultaneously spreads it, imparts an oscillation of 50 c.p.s. and directs it vertically downwards through a corona charging zone between a multiple point corona discharge electrode and a grounded target plate towards a moving belt, located a distance of 10 inches from the baffie, where it is collected in overlapping layers. The sheet is then lightly consolidated by passage between a pair of rolls under a pressure of about 10 lb./Iineal inch. The speed of the laydown receiver is adjusted to obtain a fabric weight of about 2 oz./yd.".

When a scoop element is used, it is positioned as shown in FIGS. 9 and 10 at a distances D and DJD given in Table I. The results listed in this table show that use of the scoop provides a l32l percent reduction in the number of defects/cycle, a 10- percent improvement in the percent CV of fabric weight and about 10 percent reduction in MD motion.

EXAMPLE 2 The procedure of Example 1 is repeated except that the baffle frequency is increased to 70 c.p.s. and the distance between the baffle and the laydown belt is increased to 13 inches. The results, shown in Table ll indicate a 19 percent improvement in the percent CV and about 10 percent reduction in the web defects per cycle and in the MD motion.

The procedure of Example 1 was repeated except the polymer throughput was increased to 75 lb./hr., the first orifice of the spinneret assembly had an L/D of 0.025/0.049 inch and the second orifice an L/D of 0.025/0.044 inch. Sheets were collected at various scoop settings as well as without a scoop as a control. Attempts to place the scoop at a minimum distance D less than about 0.03 inch resulted in frequent plugging and an inoperable process. The results obtained with various scoop settings are given in Table III.

It is seen that at the higher throughput, the general level of uniformity was somewhat poorer than at the lower throughputs used in the previous examples.

Run 7 shows that placing the scoop at a distance D from the target plate which is outside the critical range may give a product with poorer uniformity than the control. The remaining runs, all carried out at scoop settings within the critical limits provide improvements of from 6-22 percent in the percent CV compared to the control. Corresponding improvements are observed in the web defects and in the MD motion.

The procedure of Example 3 was repeated except that a self-relieving scoop of the type shown in FIGS. 12 and 13 was used with a baffle frequency of 60 c.p.s. and a baffle to laydown belt distance of 11 inches. Two 1.48 lbs. constant tension Negator springs (Ametek Corp., Hatfield, Pa. Spring SH6F21) were used to provide the counterbalance force which gave an equivalent 1.34 lb. load at the midpoint of the scoops. The scoop was positioned so that D,=0.063 inch and D,l =l.83. The nonwoven material collected was found to have a percent CV corrected in the range 7.47.7 and the swath width was 23 inches. When the experiment was repeated without the scoop, the percent CV corrected was 9.3 and the swath width 22 inches.

EXAMPLE A series of plexifilamentary sheets are prepared at a polymer throughput of about 140 lbs/hr. with an apparatus modified from that given in FIG. 1. Instead of forwarding the flash spun web in a horizontal direction, advancing it through a 90 turn to spread the web and then oscillating it across the receiving surface, the plexifilamentary web is formed and spread at the exit to the flash-spinning nozzle, forwarded without change of direction or oscillation parallel to a large,

flat rectangular target plate towards the receiving surface. Opposite the target plate about one inch above its lower edge and less than two inches away from the surface is a line of sharp needles spaced about one-half inch apart and projecting from a 36 inch long ion gun. A high voltage drop across the gap between the needles and the target plate provides a corona discharge field in which the plexifilamentary web is charged. Between the charging zone and the top of the target plate is a converging passage defined by the target plate and a scoop. The target plate measures about 3-6 inches in width, 8 inches in length (the direction of web flow) and 1 inch in thickness. The scoop, which is a flat rectangular bar, measures about 36 inches in width and 3 inches in length and is inclined toward the target plate so that the dimensions of the entrance and exit to the converging passage can be set and varied from test to test. In this series of tests the entrance to the converging passage, which is approximately uniform along its width (the 36-inch dimension) is varied between about 0.3 and 0.5 inch. The exit of the passage, also uniform along its width, is varied between about 0.045 inch and 0.020 inch. The width of the spread web is controlled by varying the exit dimensions of the passageway; the narrower the passageway the wider the web. Maximum web widths of about 3 feet are obtained with the scoop in place. Without the scoop, the corresponding web width is less than 2 feet. When the web width is controlled with a scoop to give about the same width as is obtained without the scoop, an improvement in the local uniformity of the deposited sheet of up to about 24 percent is obtained.

lclaim:

1. In an apparatus for forming a fibrous web that includes a means for forming a filamentary strand entrained in a gaseous stream, means for spreading the strand into a web and directing it in a path and an opposed ion gun and a target plate positioned on opposite sides of said path below said spreading and directing means, a device for confining the flow of the gaseous stream in the direction of said path comprising an element positioned on the same side of said path as but above said ion gun, said element having a surface disposed toward said plate, said surface converging toward said target plate and terminating in downstream edge spaced from said target plate, said surface and said plate forming a passage having an outlet defined by the distribution of distance between said edge and said plate, said distribution being symmetrical about the midpoint of said edge, the distance between said midpoint and said plate being in the range of from 0.03 inch to about 0.05 inch.

2. The apparatus as defined in claim 1, the ratio of the distance from a point along said edge having a straight line distance one inch from said midpoint to said plate and the midpoint distance to said plate being in the range of from 1.0 to 3.25.

3. The apparatus as defined in claim 2, where said midpoint distance is in the range from 0.03 inch to 0.25 inch.

4 The apparatus as defined in claim 3, including means for mounting said element for swinging movement toward and away from said. plate; and means for biasing said element toward said plate.

5. A process for forming fibrous sheets that includes the steps of entraining a web in a gaseous stream flowing downwardly toward a collecting means, electrostatically charging said web, and collecting said web in a tensionless state wherein the improvement comprises confining the gaseous stream flow prior to the charging step by directing said flow through a passage converging in the direction of flow to an outlet having a minimum width dimension of from 0.03 to about 0.05 inch.

6. The process of claim 5 said width dimension being in the range of from about 0.03 to 0.25 inch.

mg UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 293, 0711 Dated July 13, l w

Inventor(s) Lawrence Isakoff It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 8, lines &8 and 68, in each i'mstance,

change "0.05" to read Signed and sealed this Mi th day of March 1972.

(SEAL) Attest:

EDWARD I LF'LETCI-IER, JR. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents 

2. The apparatus as defined in claim 1, the ratio of the distance from a point along said edge having a straight line distance one inch from said midpoint to said plate and the midpoint distance to said plate being in the range of from 1.0 to 3.25.
 3. The apparatus as defined in claim 2, where said midpoint distance is in the range from 0.03 inch to 0.25 inch. 4 The apparatus as defined in claim 3, including means for mounting said element for swinging movement toward and away From said plate; and means for biasing said element toward said plate.
 5. A process for forming fibrous sheets that includes the steps of entraining a web in a gaseous stream flowing downwardly toward a collecting means, electrostatically charging said web, and collecting said web in a tensionless state wherein the improvement comprises confining the gaseous stream flow prior to the charging step by directing said flow through a passage converging in the direction of flow to an outlet having a minimum width dimension of from 0.03 to about 0.05 inch.
 6. The process of claim 5 said width dimension being in the range of from about 0.03 to 0.25 inch. 